In internet-of-things (IoT) application, the devices frequently switch between a standby mode and an operation mode. When the device enters the operation mode, the crystal oscillation circuit will restart-up and generates an accurate system clock for the system operation. Moreover, in IoT application, the low power-consumption and bill-of-materials (BOM) are major design indexes. In the conventional crystal oscillation circuit, the start-up time is often long and needs two pins to connect with an external load capacitor. As a result, both the overall system power-consumption and the BOM cost will increase.
The crystal oscillator is a collective term for oscillators of the resonant cavity type, and is often made with crystals with piezoelectric effect. Because of the property of highly selective in frequency, the crystal oscillator can generate highly accurate and highly stable frequency signals, and is widely used in microprocessor, controller, clocks, communication equipment, and so on. The main function of the crystal oscillator is to generate an original clock frequency, and by multiplying or dividing the frequency, various frequencies can be generated for various buses in a computer system. As such, the crystal oscillator is often used to substitute the LC (inductor and capacitor) resonant circuit and filters. As the crystal usually has high quality factor, light in weight and small in size, reliable, able to generate accurate and stable frequency, and so on, the crystal oscillator is often applied to devices demanding high frequency accuracy.
The known crystal oscillator usually has a start-up time in the order of milliseconds. In the systems that require frequent start-up and operate in short duration, the long start-up time occupies a large portion of system operation time, and consumes a large portion of power. As such, the long start-up time has become an issue for the contemporary applications.
FIG. 1 shows a schematic view of the structure of a conventional crystal oscillation circuit, formed by two capacitors C1, C2 and a crystal oscillator. A circuit of this type requires two pins to connect with the external load capacitor, and results in long start-up time. In other words, many cycles of oscillations must be passed before the output signal to reach pre-defined amplitude for generating a clock output. Therefore, the circuit performance is relatively restricted. Moreover, two external load capacitors (C1, C2) also add to the additional BOM cost as well as the size and area of the circuit. With the power consumption issues and the aforementioned disadvantages, it is imperative to devise a fast start-up single pin oscillation apparatus for the advance of contemporary applications.